Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $249.98   13.258%   $283.1223   $283.12   $283.10   $283.10  
  Purchase #2 $47.37   11.594%   $52.8621   $52.86   $52.85   $52.90  
  Purchase #3 $67.56   10.221%   $74.4653   $74.47   $74.45   $74.50  
  Purchase #4 $59.74   10.567%   $66.0527   $66.05   $66.05   $66.10  
  Purchase #5 $203.89   4.743%   $213.5605   $213.56   $213.55   $213.60  
  Purchase #6 $218.31   7.655%   $235.0216   $235.02   $235.00   $235.00  
  Purchase #7 $91.35   3.840%   $94.8578   $94.86   $94.85   $94.90  
  7 purchase totals = $938.20       $1,019.9423   $1,019.94   $1,019.85   $1,020.10